Project Summary/Abstract While N-linked carbohydrate is known to comprise a significant portion of the mass of HIV-1 envelope protein, the dogma has been that O-linked carbohydrate is absent from HIV-1 envelope protein. The Desrosiers laboratory has now shown unambiguously that a subset of HIV-1 isolates have O-linked carbohydrate on the V1 region of their gp120 surface glycoprotein. Furthermore, this O-linked carbohydrate is able to shield virus from recognition by potent broadly-neutralizing antibodies of the V3-glycan class. Our results suggest that long V1 regions, sometimes with sometimes without O-linked glycosylation, can emerge as a mechanism to escape neutralizing antibodies similar to the V3-glycan monoclonal antibodies that have been defined to date. Our proposed studies will better define the characteristics of V1 region sequences that are predictive of whether an individual sequence is likely to be O-glycosylated. We will determine whether there is specificity to the blocking effects, i.e. whether one O-glycosylated V1 region potently blocks recognition by one V3-glycan mAb but not another while a different O-glycosylated V1 region may have different specificities to its blocking effects. We will use experimental SHIV infection of rhesus monkeys to examine whether there is selective disadvantage to such long O-glycosylated V1 regions in the absence of such antibodies and to examine the types of selective pressure that may drive elongation and O-glycosylation of V1 regions.